The present embodiments relate to determining a tissue parameter.
In angiography, it is known to administer contrast agent that may be clearly identified in images (e.g., x-ray images) and therefore allows an assessment of the blood flow and the passage of blood through the vascular system of a patient and also through the tissue. One conventional method for tracking the propagation of the contrast agent is digital subtraction angiography. With this, a mask image is recorded without contrast agent. After this, raw images are recorded, frequently as a temporal series in a time range during which the contrast agent passes through the target region or vascular system of interest. The contrast agent may also be seen in these raw images. In order to remove the anatomy causing interference during the analysis, digital subtraction angiography x-ray images are produced by removing the mask image from the raw image so that image information that essentially only relates to the contrast agent remains. In order to achieve better orientation, two-dimensional digital subtraction angiography may be performed from a number of projection directions (e.g., using a biplane system). This may provide raw images and mask images of the target region from, for example, perpendicular projection directions, so that x-ray images are obtained for these perpendicular projection directions and may be viewed and analyzed for diagnosis purposes.
Two-dimensional subtraction angiography-x-ray images may be used to determine a plurality of time parameters, which show the behavior of the contrast agent or are derived therefrom, when the time-intensity curves TIC in the x-ray images are considered. In this process, the image data (e.g., intensity) of the x-ray image for all the time points of the series for which an x-ray image is available is plotted against recording times, at least for some image points/regions of interest, producing a time/intensity curve that may also be referred to as a contrast agent curve. This is accessible for conventional analysis methods. For example, the time up to the maximum contrast agent concentration at the image point may therefore be considered (e.g., the “time to peak” (TTP)). A further, frequently considered time parameter is the mean transit time MTT, which may be defined in different ways (e.g., relative to the maximum value of the time/intensity curve).
Contrast agent-assisted examinations are also particularly important with respect to the human brain (e.g., with regard to the passage of blood through the parenchyma). In order to examine this, regions of interest (ROI) may be defined in the subtraction angiography-x-ray images, which are as far as possible not overlaid by larger vessels. Integration over the time/intensity curve provides information relating to the quantity of contrast agent that has flowed through at an image point. When the time/intensity curve is considered for an entire region of interest, this applies to all the structures through which the x-rays pass there. This allows the cerebral blood volume (CBV) and the cerebral blood flow (CBF) to be derived (e.g., relative to a reference region). Specifically, the procedure may be described such that the area A below the time/intensity curve I(t) is first determined as
  A  =            ∫      0      T        ⁢                  I        ⁡                  (          t          )                    ⁢                        ⅆ          t                .            
The contrast agent concentration C is now proportional to A. The cerebral blood volume CBV is defined as proportional to
      C    ⁢                  ⁢    B    ⁢                  ⁢    V    ∝                    C        ROI                    Cy        Artery              .  
If a reference region (reference ROI) is now added, the resulting relative blood volume rCBV is
      r    ⁢                  ⁢    C    ⁢                  ⁢    B    ⁢                  ⁢    V    ∝                    A        ROI                    A        ref              .  
The reference ROI here does not have to originate from the x-ray image itself, with “left/right” comparisons also frequently being performed, but may also originate from an angiography data record and the like recorded before an intervention. The corresponding rCBF may also be determined using a suitable conversion.
In the prior art, tools were already known to help a user with the analysis of two-dimensional subtraction angiography-x-ray images (e.g., for images recorded using a biplane x-ray facility).